The invention relates to a communication system comprising at least a master station and a plurality of terminals intended to communicate with the master station via a transmission medium, said terminals comprising means for requesting the assignment of a certain number of reservations on the transmission medium. The invention also relates to a terminal intended to be used in such a system, and a request method permitting such a terminal to be assigned a certain number of reservations on a transmission medium.
The invention is notably applied to interactive satellite broadcast systems.
The article by Nedo Celandroni and Erina Ferro, entitled xe2x80x9cThe FODA-TDMA Satellite Access Scheme: Presentation, Study of the system, and Resultsxe2x80x9d and published in IEEE Transactions on Communications, vol. 39, no. 12, in December 1991, describes a communication system in which a plurality of terminals can access a transmission medium via a time-division multiple access mechanism for communication with a master station. This mechanism comprises a sub-division of time into time intervals, which are dynamically assigned to the terminals at the request of these terminals. The terminals thus transmit requests which indicate the number R of time intervals they need to have at a given instant. Each terminal calculates this number R based on the number of packets present in its transmission queue the moment the terminal formulates its request.
It is an object of the invention to propose a method of optimizing the request mechanism of the terminals in order to limit the number of reservations needlessly requested by the terminals. The invention thus enables to obtain a greater efficiency in the sharing of resources, which is particularly important in the case where the transmission medium is a satellite channel, because this type of medium is particularly costly.
This object is achieved with a communication system, a terminal and a request method as respectively described in the claims 1, 2 and 3 of the present application.
The invention takes the existence into account of a transmission delay which is inherent in any communication system (and particularly large in the case of transmissions by satellite). This delay is subject to jitter. The master station is thus to take the maximum value of the transmission delay into account when it transmits reservation assignments to the terminals: these assignments are to be sent sufficiently early to arrive before they expire (that is to say, before the start of the corresponding time interval). The assignments arriving early are stored in an assignment memory of the terminal until their date of expiration. The invention comprises taking assignments stored in the assignment memory of the terminal into account for calculating the number of reservations to be requested at a given instant. This strategy enables to avoid that reservations that have already been assigned by the master station are requested again. The invention thus significantly reduces the number of assigned reservations that are ultimately not used by the terminals.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.